Tuning Thermal Transport: Investigating Defect Shape Variations in Silicon Phononic Crystals

In this work we investigate the lattice thermal conductivity (LTC) of 2D Si Phononic Crystals (PnCs), which are created by introducing a periodic array of defects on pristine membranes. In literature, the introduction of such defects in Si membranes shows the possibility of opening phonon band gaps and reducing the material’s LTC. Here, we evaluated how the shape of periodic defects (circles, squares and triangles) influences the LTC reduction in those systems. For that, we used homogeneous non equilibrium molecular dynamics (HNEMD), as implemented in GPUMD, together with the Tersoff potential for Si. We found that LTC decreases logarithmically with defect area, regardless of the defect’s shape. Furthermore, we show that, for those PnCs, thermal conductivity is primarily governed by the neck size, the smallest distance between defects. A plot of κ versus neck size collapses all data points onto the same trend, again irrespectively of defect shape. The directional dependence shows a power-law relationship (κ ∝ n^1.8) along [110] and an exponential decay along [-110], where we attribute the functional difference between the former and the latter to the 2x1 surface reconstruction of dimers in our structures. We conclude that neck size, not defect shape, is the dominant mechanism for LTC reduction in these Si PnCs.